The present invention relates to acoustical logging instruments, and particularly, acoustical logging instruments designed to detect the presence of vertical fractures in formations surrounding boreholes. The detection of vertical fractures in formations surrounding the borehole is important in the petroleum industry since many formations produce from fractured reservoirs. In normal drilling operations, the drilling mud used in drilling the borehole will invade vertical fractures, and prevent the location of the fractured formations using normal or conventional logging tools.
In the past, attempts have been made to provide acoustical logging instruments which would produce acoustic waves that would travel circumferentially around the borehole to detectors where the acoustical waves were received. The signals produced by the received waves were separately reported. Normally, the logging instruments were constructed to detect both the compressional and shear waves traveling circumferentially around the borehole wall. Both compressional and shear waves were detected since compressional waves would pass through a fluid-filled fracture while shear waves would be absorbed in the fluid filling the fracture. Thus, if a compressional wave was present but no shear was present, it would be an indication that a vertical fracture was present in the formation.
The prior attempts to instrument the above conception have suffered from certain disadvantages. For example, one approach, described in U.S. Pat. No. 3,794,976, was used with omnidirectional transmitting transducers to produce acoustical waves which traveled through the formation. The received waves were then processed to separate the compressional and shear waves which could then be displayed. This approach was not entirely successful, possibly due to the fact that the transmitting transducer was located a considerable distance from the surface of the borehole wall. Another approach, as described in U.S. Pat. No. 3,775,739, utilized two sets of transmitting transducers. One of the transmitting transducers was oriented to produce substantially compressional waves in the formation while the other transducer was angled so as to produce substantially shear waves in the formation. This approach is a considerable improvement over the single omni-directional transducer in that the shear wave and compressional waves were separately produced and received. While this approach improved the results, it also included the additional complication of having two transmitting transducers and two separate receiving transducers in place of a single-transmitting and a single-receiving transducer.